We are attempting to delineate the role of phagocytic leukocytes (neutrophils and monocytes) in resistance to opportunistic mycoses in man. The study entails the establishment of quantitative methods for investigating the fungicidal activity of these leukocytes in vitro and comparing the function of leukocytes from normal subjects and from those with mycoses or conditions known to predispose to opportunistic infections. At present, these tests employ candida species; in the future, other yeast-phase fungi (Torulopsis, Cryptococcus) will be examined. An attempt is being made to discern the actual mechanisms of fungicidal activity by phagocytes. The role of myeloperoxidase and of other lysosomal components is being investigated.